In different parts of electrical transformers, insulating material is used to avoid flash-overs and such. This insulating material is typically cellulose based since such a paper or pressboard material is cheap and easy to handle while giving adequate insulation. Examples of insulators in an oil filled transformer are:                Spacers, positioned between the turns/discs of a winding, allowing oil to circulate there between.        axial sticks, positioned between the winding and the core, or between different windings.        cylinders positioned around a winding, between the a winding and its core, or between different windings.        winding tables, positioned atop and below the plurality of windings, supporting the same.        insulation coating of the conductor of the windings.        
However, cellulose typically has a moisture content of about 6-8% by weight. Although insulation materials are dried during transformer manufacturing, it is well known that moisture which is present in cellulose insulation continues to be a major cause of problems (dielectric, thermal, ageing, bubble formation and unreliability in operation) during operational life of transformer. The shrinkage of spacers due to moisture may result in axial imbalance of windings if not balanced perfectly which will lead to higher short circuit forces.
Further, cellulose cannot withstand high operating temperatures of the transformer above 105° C. over long time. It would be desirable to be able to operate a transformer continuously at high temperature or to be able to withstand overloads, but then the cellulose based insulators now used are not suitable. Meta-aramid e.g. Nomex™ is an insulating material with higher heat resistance than cellulose, but it is also porous and contains a high amount of moisture.
The high moisture uptake sensitivity to high temperatures of cellulose results in more rapid ageing of the insulating cellulose material. For instance, thermal deterioration of paper is directly proportional to its water content, for each 6-8° C. rise in temperature, the life of paper insulation is halved.
In order to be durable and withstand the strain put on it, especially in oil at high temperatures over long periods of time, the insulating material also needs to be strong and resilient.
U.S. Pat. No. 8,085,120 discloses an insulation system for a fluid-filled power transformer that allows for operation of the transformer at higher temperatures and with lowered susceptibility to aging. The insulation system includes a plurality of fibres that are bound together by a solid binding agent. The solid binding agent may, for example, form sheaths around the fibres or may be in the form of dispersed particles that bind the fibres to each other. A pair of base fibres each has a sheet of binder material which are bound to each other, binding the two base fibres to each other. The fibres form a porous insulating lattice which allows the cooling fluid of the transformer to penetrate and circulate through the insulating material, thereby avoiding bubbles where partial discharges may occur.
WO 2004/072994 discloses a transformer which includes a core defining a core window, a first coil surrounding a portion of the core and including a portion located within the core window, a second coil surrounding a portion of the core and including a portion located within the core window, and a polymer barrier insulation member that is located at least partially within the core window and positioned between the first coil and the second coil. The insulation material is made of a high temperature polymer configured to withstand an operating temperature of approximately 130° C. Further, the polymer material is configured so as to absorb no more than approximately 1% moisture.